The inventions concern methods for the manufacture of semiconductor devices, such as thin film transistors (TFT) or thin film integrated circuits in which these are employed, and especially the thin film integrated circuits for active type liquid crystal display devices (liquid crystal displays) for example, which are established on an insulating substrate such as glass for example, or on an insulating film which has been formed on various types of substrate, wherein silicon oxide films which have been formed by means of a PVD method or a CVD method are used as gate insulating films, and, they concern in particular methods of heat treatment for gate insulating films, and heat treatment apparatus, for obtaining gate insulating films which have good characteristics.
Semiconductor devices wherein TFT are established on an insulating substrate such as glass, such as image sensors and active type liquid crystal display apparatus in which TFT are used to drive the picture elements for example, have been developed in recent years.
TFT in which silicon semiconductors are used in the form of a thin film have generally been used for the TFT in such devices. The thin film silicon semiconductors can be broadly classified into two types, namely those consisting of an amorphous silicon semiconductor and those consisting of a silicon semiconductor which has crystallinity. The amorphous silicon semiconductors can be manufactured comparatively easily with a gas phase method, having a low manufacturing temperature, and they are suitable for mass production, and so they are used most generally, but their properties, such as their electrical conductivity for example, are poor when compared with those of the silicon semiconductors which have crystallinity and so, in the future, there will be a considerable demand for the establishment of a method for the manufacture of TFT consisting of silicon semiconductors which have crystallinity in order to attain higher speeds.
The properties of the gate insulating film do not present much of a problem in the case of a TFT where amorphous silicon which has a small mobility has been used. For example, a silicon nitride film which has poor electrical characteristics when compared with silicon oxide can be used for the gate insulating film in a TFT in which amorphous silicon has been used. However, with a TFT in which a crystalline silicon film which has a high mobility is used, the characteristics of the gate insulating film which are about the same as those of the silicon film itself present a major problem.
A thermal oxide film is preferred for the gate insulating film. For example, a gate insulating film can be obtained using the thermal oxidation method if the substrate can withstand high temperatures, being a quartz substrate for example. (For example, JP-B-H3-71793) (The term xe2x80x9cJP-Bxe2x80x9d as used herein signifies an xe2x80x9cexamined Japanese patent publicationxe2x80x9d.)
A high temperature of at least 950xc2x0 C. is required to obtain a silicon oxide film which is satisfactory for use as a gate insulating film using the thermal oxidation method. However, there are no other substrates except quartz which can withstand such high temperature treatment, and quartz substrates are expensive and, moreover, there has been a problem in that the production of larger areas has been difficult because the melting point is so high.
However, less expensive glass substrate materials have the problem that their distortion point is less than 750xc2x0 C., usually 550-650xc2x0 C., and thus the substrate cannot withstand high temperatures required to obtain a thermal oxide layer using normal methods. Consequently, gate insulating films have been formed using the physical gas phase growing methods (PVD methods, for example the sputter method) and chemical gas phase growing methods (CVD methods, for example the plasma CVD and photo CVD methods) with which they can be formed at lower temperatures.
However, insulating films which have been manufactured using PVD methods or CVD methods have a high concentration of hydrogen and unpaired bonds, for example, and furthermore the boundary surface characteristics are not good. They are therefore also weak in respect of the implantation of hot carriers for example, and centers for charge capture are easily formed, originating from the unpaired bonds and hydrogen. Consequently, when these films are used as gate insulating films for TFT, there is a problem in that the electric field mobility and the sub-threshold characteristic value (S value) are not good, or there are problems in that the gate electrode leakage current increases and the ON current is reduced (deterioration, change with the passage of time).
For example, generally, in those cases where the sputter method which is a PVD method is used, a film of a compound of essentially just oxygen and silicon is formed in principle if a synthetic quartz target comprising oxygen and silicon of high purity is used for the target. However, it is very difficult to obtain a silicon oxide film in which the proportions of oxygen and silicon in the film obtained are close to the stoichiometric ratio and in which there are few unpaired bonds. For example, oxygen is preferred as the sputter gas. However, oxygen has a low atomic weight and so the sputter rate (the accumulation rate) is low, and it is inappropriate as a sputter gas when mass production is being considered.
Furthermore, although a satisfactory rate of film formation can be obtained in an atmosphere of argon, for example, the proportions of oxygen and silicon differ from the stoichiometric ratio and the material obtained is very inappropriate as a gate insulating film.
Moreover, it is difficult to reduce the number of unpaired silicon bonds whatever the sputtering atmosphere, and the unpaired silicon bonds Si. or SiO. must be stabilized as Sixe2x80x94H and Sixe2x80x94OH by carrying out a heat treatment in a hydrogen atmosphere after film formation. However, the Sixe2x80x94H and Sixe2x80x94OH bonds are unstable and they are easily broken by accelerated electrons, and they inevitably revert back into unpaired silicon bonds. The presence of the weak Sixe2x80x94H and Sixe2x80x94OH bonds is the cause of the deterioration which is caused by hot carrier implantation mentioned above.
Similarly, a large amount of water in the form of Sixe2x80x94H and Sixe2x80x94OH is included in a silicon oxide film which has been manufactured using the plasma CVD method, and there are many unpaired bonds, and this also gives rise to the problems mentioned above. In addition, in those cases where tetraethoxysilane (TEOS) has been used as a source of silicon which can be handled comparatively easily, there is a problem in that a high concentration of carbon is included in the silicon oxide film. The present invention provides a means of resolving the problems described above.
It is known that silicon oxide films which have a low hydrogen concentration within the silicon oxide film, in which the nitrogen concentration within the silicon oxide film is increased, and which are ideal as gate insulating films can be obtained by subjecting silicon oxide films, for example silicon oxide films which have been formed by thermal oxidation, to a heat treatment at a temperature of at least 900xc2x0 C. in an atmosphere of dinitrogen monoxide (N2O).
Furthermore, according to research carried out by the inventors, a comparatively high concentration of carbon was included in silicon oxide films which have been formed using the plasma CVD method with TEOS as a raw material, but it was clear that the carbon in the silicon oxide was oxidized and eliminated from the silicon oxide film as carbon dioxide gas on heat treating at a temperature of at least 900xc2x0 C. in an N2O atmosphere in the same way as described above.
However, the heat treatments carried out at this time are at a high temperature of at least 900xc2x0 C. and so the process is only possible with substrates which have a high distortion point, such as quartz substrates. Consequently, the heat treatment cannot be introduced into the low temperature processes in which TFTs are formed using various glass substrates which have a distortion point below 750xc2x0 C., and typically of 550-650xc2x0 C.
The inventors have carried out research with a view to lowering the temperature of this reaction, and they have discovered that an effect similar to that obtained on heat treating at a temperature of at least 900xc2x0 C. can be obtained on heat treating at 300-700xc2x0 C., and preferably at 500-600xc2x0 C., by irradiating with ultraviolet light during heat treatment in an N2O atmosphere. The wavelength of the ultraviolet light used at this time is set to 100-350 nm, and preferably to 150-300 nm.
The first invention is characterized by the fact that a silicon oxide film is improved so as to be satisfactory for use as a gate insulating film by heat treating a silicon oxide film, which has been formed by a PVD method or a CVD method, in an N2O atmosphere at 300-700xc2x0 C., and preferably at 500-600xc2x0 C., and irradiating with ultraviolet light at the same time. Heat treatment at 300-700xc2x0 C., and preferably at 500-600xc2x0 C. in a hydrogen atmosphere, or hydrogen nitride atmosphere such as an ammonia (NH3) or hydrazine (N2H4) atmosphere, may be carried out prior to the abovementioned heat treatment/ultraviolet light irradiation process. Furthermore, irradiation with ultraviolet light in the same way as in the N2O atmosphere may be carried out in the heating process in a hydrogen or hydrogen nitride atmosphere.
The duration of the heat treatment in an N2O atmosphere depends on the characteristics of the silicon oxide film, the heat treatment temperature and the intensity of the ultraviolet light, for example, but, in consideration of mass production, it is preferably set to from 30 minutes to 6 hours. Furthermore, the rates of raising and lowering the substrate temperature in the heat treatment process should be determined for the execution of the invention but, in consideration of mass production, the rate at which the temperature is raised and the cooling rate are preferably from 5 to 30xc2x0 C./min. Furthermore, the raising of the temperature and cooling may be carried out in a nitrogen atmosphere.
Furthermore, it is known that by subjecting silicon oxide films, for example silicon oxide films which have been formed by means of the thermal oxidation method, to a heat treatment at a temperature of at least 900xc2x0 C. in an atmosphere of a hydrogen nitride, such as ammonia (NH3) or hydrazine (N2H4) for example, nitriding is effected and the number of unpaired bonds is reduced, thereby increasing the concentration of nitrogen in the silicon oxide film and making it possible to obtain silicon oxide films which are ideal as gate insulating films.
However, the heat treatments carried out at this time are at a high temperature of at least 900xc2x0 C. and so the process is only possible with substrates which have a high distortion point such as quartz substrates. Consequently, the heat treatment cannot be introduced into the low temperature processes in which the TFTs are formed using various glass substrates which have a distortion point below 750xc2x0 C., and typically of 550-650xc2x0 C.
The inventors have carried out research with a view to lowering the temperature of this reaction and they have discovered that the same effect as that obtained by carrying out a heat treatment at a temperature of at least 900xc2x0 C. can be obtained with heat treatment at 300-700xc2x0 C., and preferably at 500-600xc2x0 C., by irradiating with ultraviolet light during a heat treatment in an NH3 or N2H4 atmosphere. The wavelength of the ultraviolet light used at this time is set to 100-350 nm, and preferably to 150-300 nm.
The second invention is characterized by the fact that a silicon oxide film is improved so as to be satisfactory for use as a gate insulating film by heat treating a silicon oxide film, which has been formed by a PVD method or a CVD method, in a hydrogen nitride, such as NH3 or N2H4, atmosphere at 300-700xc2x0 C., and preferably at 500-600xc2x0 C., and irradiating with ultraviolet light at the same time.
The duration of the heat treatment in the hydrogen nitride atmosphere depends on the characteristics of the silicon oxide film, the heat treatment temperature and the intensity of the ultraviolet light, for example, but, in consideration of mass production, it is preferably from 30 minutes to 6 hours. Furthermore, the rates of raising and lowering the substrate temperature in the heat treatment process should be determined for the execution of the invention but, in consideration of mass production, the rate at which the temperature is raised and the cooling rate are preferably from 5 to 30xc2x0 C./min. Furthermore, the raising of the temperature and cooling may be carried out in a nitrogen atmosphere.
The third invention involves carrying out the heat treatment of a silicon oxide film which has been accumulated on an active layer, by means of a CVD method or a PVD method, at 300-700xc2x0 C., and preferably at 500-600xc2x0 C., in an atmosphere of dinitrogen monoxide (N2O) while irradiating with ultraviolet light, and then replacing the atmosphere with a hydrogen nitride, such as ammonia (NH3) or hydrazine (N2H4) for example, atmosphere and carrying out heat treatment at 300-700xc2x0 C., and preferably at 500-600xc2x0 C., in the said atmosphere while irradiating with ultraviolet light. At this time the wavelength of the ultraviolet light used is set to 100-350 nm, and preferably 150-300 nm.
In those cases where the abovementioned processes are carried out in a single reaction chamber it is necessary to change the atmosphere from N2O to hydrogen nitride. At this time it is desirable that the hydrogen nitride should be introduced after the N2O has been reduced to a satisfactorily low concentration. This is because there is a serious danger of explosion if N2O and hydrogen nitride are mixed together. Consequently, it is best if the hydrogen nitride is introduced after first evacuating the N2O atmosphere from the chamber. More easily, the N2O can be displaced with nitrogen to provide a nitrogen atmosphere, reducing the concentration of N2O satisfactorily, and the hydrogen nitride can be introduced subsequently.
In the abovementioned process involving two heat treatments the temperature may be raised and lowered for each of the heat treatment processes, or the temperature may be held essentially constant. Similarly, the irradiation with ultraviolet light may be started and stopped for each heat treatment in the two heat treatment processes, or it may be irradiated continuously.
The durations of the heat treatments in the N2O and hydrogen nitride atmospheres depend on the characteristics of the silicon oxide film, the heat treatment temperature and the intensity of the ultraviolet light, for example, but, in consideration of mass production, they are preferably from 30 minutes to 6 hours. Furthermore, the rates of raising and lowering the substrate temperature in the heat treatment processes should be determined for the execution of the invention but, in consideration of mass production, the rates at which the temperature is raised and the cooling rates are preferably from 5 to 30xc2x0 C./min. Furthermore, the raising of the temperature and cooling may be carried out in a nitrogen atmosphere.
The fourth invention involves carrying out the heat treatment of a silicon oxide film which has been accumulated on an active layer, by means of a CVD method or a PVD method, at 300-700xc2x0 C., and preferably at 500-600xc2x0 C. in a hydrogen nitride atmosphere while irradiating with ultraviolet light and then replacing the atmosphere with an N2O atmosphere and carrying out a heat treatment at 300-700xc2x0 C., and preferably at 500-600xc2x0 C., in the said atmosphere while irradiating with ultraviolet light. At this time the wavelength of the ultraviolet light used is 100-350 nm, and preferably 150-300 nm.
The same precautions as in the third invention described above must be taken when changing the atmosphere from hydrogen nitride to N2O in the process described above. Furthermore, the raising and lowering of the temperature and the irradiation and interruption of the irradiation with ultraviolet light should be controlled in the same way as in the third invention. The durations of the heat treatments are also the same.
In this present invention, for example, the sputter method should be used as the PVD method and the plasma CVD method, the low pressure CVD method or the atmospheric pressure CVD method should be used as the CVD method. Other methods of film formation can also be used. Furthermore, methods in which TEOS is used as a raw material can also be used in the plasma CVD method or the low pressure CVD method. In the former case TEOS and oxygen are used for the raw material gas and the accumulation should be carried out at a substrate temperature of 200-500xc2x0 C. In the latter case a silicon oxide film which is undamaged by the plasma can be obtained at a comparatively low temperature (for example at 375xc2x0C.xc2x120xc2x0 C.) using TEOS and ozone as raw materials.
Similarly, with the low pressure CVD method it is possible to reduce plasma damage of the active layer if monosilane (SiH4) and oxygen gas (O2) are used as the principal raw materials. Furthermore, the ECR-CVD method in which a discharge under ECR (electron cyclotron resonance) conditions is used from among the plasma CVD methods gives rise to less damage by the plasma and so it is possible to form even better gate insulating films.
The fifth invention concerns heat treatment apparatus which is appropriate for the execution of the abovementioned processes, and it provides heat treatment apparatus which is characterized by having a chamber for heat treatment purposes, a standby chamber in which the substrate is held before carrying out the heat treatment and in which the substrate is held after carrying out heat treatment, and a front chamber which is furnished with a transporting device for moving the substrate, in that a substrate holder which is furnished with a heater which heats the substrate is provided in the chamber for heat treatment purposes, and in that a light source for irradiating the substrate with ultraviolet light is provided outside or inside the chamber for heating the aforementioned substrate.
In order to provide a further increase in productivity, the substrate holder inside the chamber in this apparatus may be a generally (roughly) conveyor-like transporting device which is made of a heat resistant metal, and the heat treatment can be carried out while the substrate is being moved. Furthermore, the substrate holder in the chamber for heating the substrate may take the form of a generally conveyor-like transporting device which is made of a heat resistant metal, and a plurality of substrates may be taken up and heat treated at the same time. Moreover, a heater may be established in the lower part of the generally conveyor-like transporting device.
Other apparatus of the invention has a cylindrical chamber with heaters established around the perimeter of the aforementioned cylindrical chamber for heating the substrates, and a light source for irradiating the substrates with ultraviolet light is established in the center of the aforementioned cylindrical chamber, and the construction is such that substrates are taken up in such a way as to be arranged along the inner wall of the aforementioned cylindrical chamber. With such an arrangement, the ultraviolet light can be utilized effectively and productivity can be improved.
A reaction chamber which has a device with which the atmosphere can be controlled and a device for ultraviolet light irradiation is required for the execution of the third and fourth inventions described above. In practical terms, this is heat treatment apparatus which is characterized by having a chamber for heat treatment, a standby chamber in which the substrates are held before carrying out the heat treatment and the substrates are held after carrying out heat treatment, and a front chamber which is furnished with a transporting device for moving the substrate, in that a substrate holder which is furnished with a heater which heats the substrate is provided in the chamber for heat treatment, and in that a light source for irradiating the substrate with ultraviolet light is provided outside or inside the chamber for heating the aforementioned substrates. Furthermore, it may have a plurality of chambers so that the heat treatment in an N2O atmosphere and the heat treatment in a hydrogen nitride atmosphere can be carried out in different chambers.
In order to provide a further increase in productivity, the substrate holder inside the chamber in this apparatus may be a transporting device such as a generally conveyor-like system which is made of a heat resistant metal, and the heat treatment can be carried out while the substrate is being moved. Furthermore, the substrate holder in the chamber for heating the substrate may take the form of a generally conveyor-like transporting device which is made of a heat resistant metal, and a plurality of substrates may be taken up and heat treated at the same time. Moreover, a heater may be established in the lower part of the outline conveyor-like transporting device.
When a silicon oxide film which has been formed by means of a CVD method or a PVD method is heat treated in an N2O atmosphere at a temperature of 900xc2x0 C. or above, unpaired bonds may be taken up with nitrogen, and the Sixe2x80x94H bonds and Sixe2x80x94OH bonds in the silicon oxide film are converted to the nitride or the oxide, becoming Sixe2x89xa1N or Si2xe2x95x90Nxe2x80x94O bonds, and the hydrogen content in the silicon oxide film is reduced. These reactions are liable to proceed in particular at the boundary between the silicon oxide and silicon and, as a result, the nitrogen is concentrated at the silicon oxide/silicon boundary. The amount of nitrogen which is added and concentrated close to the boundary with such a technique is at least ten times the average concentration in the silicon oxide film. The inclusion of 0.1-10 atom. %, and typically the inclusion of 1-5 atom. %, of nitrogen in the silicon oxide is desirable for a gate insulating film.
Consequently, the unpaired bonds and the Sixe2x80x94H bonds and Sixe2x80x94OH bonds which are weak bonds and easily broken by hot carriers at the boundary between the gate insulating film and the active layer are converted to Sixe2x89xa1N bonds and Si2xe2x89xa1Nxe2x80x94O bonds, for example, which are strong bonds, and the extent of any change in the chemical state due to hot carriers is greatly reduced.
In this way, the durability with respect to hot carriers is improved by converting the unpaired bonds, Sixe2x80x94H bonds and Sixe2x80x94OH bonds in the silicon oxide film, and especially in the vicinity of the boundary with the silicon film, to nitride or oxide, and when the silicon oxide film was used as a gate insulating film for a TFT, the effect was to improve the electric field mobility and the sub-threshold characteristic value (S value) and to prevent any falloff in the ON current (deterioration, change with the passage of time).
Reactions of the type indicated above proceed only on heat treatment at a temperature of at least 900xc2x0 C. It has been concluded that this is principally because the temperature required to break down N2O is at least 900xc2x0 C. However, the temperature can be reduced if irradiation with ultraviolet light is used conjointly. The wavelength of the ultraviolet light used at this time is 100-350 nm, and preferably 150-300 nm. It has been concluded that this is because a high temperature as described above is not required since the N2O is broken down by the ultraviolet light, and the same reactions as indicated above can proceed even on heat treatment at 300-700xc2x0 C., and preferably at 500-600xc2x0 C. Furthermore, the Sixe2x80x94OH bonds, Sixe2x80x94H bonds and the unpaired bonds in particular in a silicon oxide film which is being irradiated with ultraviolet light readily absorb the ultraviolet light and, as a result, a state of chemical excitation arises in these parts, and it is thought that this also promotes chemical reaction. The facts indicated above have been readily verified by means of the experiments described below.
Samples where a silicon oxide film had been formed with a thickness of 1200 xc3x85 using the plasma CVD method with TEOS and oxygen as raw materials on a silicon wafer were used in the experiments. The sample was heat treated in an N2O atmosphere while being irradiated with ultraviolet light and the nitrogen concentration was then investigated using the secondary ion mass spectroscopy (SIMS) method. The results obtained are shown in FIG. 9. Here, FIG. 9(A) is the concentration profile in the depth direction of a sample which had been heat treated for 3 hours at 400xc2x0 C. in an N2O atmosphere with conjoint irradiation with ultraviolet light. For comparison, the concentration profile in the depth direction of the sample before annealing is shown in FIG. 9(B).
From this analysis it is confirmed that on looking at a sample which has been annealed at 400xc2x0 C. in a dinitrogen monoxide atmosphere with the conjoint use of irradiation with ultraviolet light shown in FIG. 9(A), the nitrogen concentration at the boundary between the silicon oxide and the silicon is higher by an order of magnitude when compared with the sample before carrying out the anneal.
Moreover, the unpaired bonds of the silicon are difficult to convert to nitride or oxide with the abovementioned ultraviolet light irradiation and heat treatment in an N2O atmosphere. By heat treating at a suitable temperature (300-700xc2x0 C., and preferably 500-600xc2x0 C.) in an atmosphere of hydrogen or a hydrogen nitride, such as ammonia (NH3) or hydrazine (N2H4) for example, the unpaired bonds Si. may be converted to Sixe2x80x94H bonds in order to promote reaction. The reaction is facilitated if irradiation with ultraviolet light is carried out at this time. Stable bonds can then be obtained by means of the reaction described above if heat treatment in an N2O atmosphere/ultraviolet irradiation process is carried out subsequently. Moreover, on treatment in a hydrogen nitride atmosphere, the Sixe2x80x94H bonds and Sixe2x95x90O bonds are converted to the nitride and form Si N or Sixe2x80x94Nxe2x95x90H2 bonds.
The effect is especially pronounced in those cases where the invention is applied to silicon oxide films which have been formed using the sputter method (and especially the silicon oxide films in which the oxygen concentration is less than the stoichiometric ratio obtained with argon, for example, as the sputter atmosphere). This is because the deficient oxygen can be supplemented by heat treating such a film in an N2O atmosphere and the composition of the silicon oxide film can be made to approach the stoichiometric ratio.
A silicon oxide film which has been formed using a sputter method of this type can be subjected to a heat treatment at a suitable temperature in an atmosphere of hydrogen or a hydrogen nitride, such as ammonia (NH3) or hydrazine (N2H4) for example, and the unpaired bonds Si. can be converted to Sixe2x80x94H bonds before carrying out the heat treatment in an N2O atmosphere. The oxidation by heat treatment in an N2O atmosphere is further facilitated by this means.
The facts outlined above show that the formation of silicon oxide films by means of the sputter method is not disadvantageous. That is to say, conventionally, the formation of silicon oxide films by the sputter method has only been carried out under limited atmospheric conditions to provide a composition approaching the stoichiometric ratio. For example, when mixtures of oxygen and argon have been considered for the atmosphere, the condition that oxygen/argon greater than 1 had to be fulfilled and, for preference, it has been desirable that the treatment should be carried out in a pure oxygen atmosphere. Consequently, the rate of film formation has been low and this has not been suitable for mass production. Furthermore, oxygen is a reactive gas and problems have arisen with oxidation of the vacuum apparatus and the chamber for example.
However, with this present invention, even silicon oxide films which have a composition far remote from the stoichiometric composition can be converted to silicon oxide films which are suitable for use as gate insulating films, and so even with the same oxygen/argon mixed atmospheres the reaction can be carried out under useful conditions in terms of the rate of film formation such that the oxygen/argon ratio is less than or equal to 1. For example, the rate of film formation is very high with an atmosphere of pure argon, and film formation can be achieved under stable conditions.
The invention provides a special effect when it is applied to silicon oxide films which have been formed by means of the plasma CVD method using a silicon source which contains carbon, such as TEOS for example. Carbon is included in large amounts in these silicon oxide films, and the carbon which is present at the boundary with the silicon film in particular causes a fall off of the TFT characteristics. Oxidation is promoted by heat treatment in an N2O atmosphere in this invention, and the carbon is also oxidized at this time and released from the system as carbon dioxide gas, and so the carbon content of the film can be reduced.
As a result, by making use of the present invention, the hydrogen and carbon concentrations in a silicon oxide film which has been formed by the plasma CVD method with TEOS as a raw material gas can be reduced, and the nitrogen concentration can be increased, while maintaining a low temperature of 300-700xc2x0 C. Thus, TFT in which such a silicon oxide film is used as a gate insulating film exhibit excellent characteristics and high reliability.
When a silicon oxide film which has been formed by means of a CVD method or a PVD method is heat treated in an NH3 or N2H4 atmosphere at a temperature of 900xc2x0 C. or above, unpaired bonds may be taken up with nitrogen, and the Sixe2x80x94H bonds and Sixe2x80x94OH bonds in the silicon oxide film are converted to the nitride, or the oxide, becoming Sixe2x89xa1N or Si2xe2x95x90Nxe2x80x94H bonds, and the nitrogen content in the silicon oxide film is increased. In particular, this reaction is liable to proceed at the boundary between the silicon oxide and silicon and, as a result, the nitrogen is concentrated at the silicon oxidexe2x80x94silicon boundary. The amount of nitrogen which is added and concentrated close to the boundary with such a technique is at least ten times the average concentration in the silicon oxide film. The inclusion of 0.1-10 atom. %, and typically the inclusion of 1-5 atom. %, of nitrogen in the silicon oxide is desirable for a gate insulating film.
Consequently, the unpaired bonds and the Sixe2x80x94H bonds and Sixe2x80x94OH bonds which are weak bonds and easily broken by hot carriers at the boundary between the gate insulating film and the active layer are converted to Sixe2x89xa1N bonds and Si2xe2x95x90Nxe2x80x94O bonds, for example, which are strong bonds, and the change in the chemical state due to hot carriers is greatly reduced.
In this way, the durability with respect to hot carriers is improved by converting the unpaired bonds, Sixe2x80x94H bonds and Sixe2x80x94OH bonds in the silicon oxide film, and especially in the vicinity of the boundary with the silicon film, to nitride or oxide, and when used as a gate insulating film for a TFT, the effect is to improve the electric field mobility and the sub-threshold characteristic value (S value) and to prevent any falloff in the ON current (deterioration, change with the passage of time).
Reactions of the type indicated above proceed only with heat treatment at a temperature of at least 900xc2x0 C. It has been concluded that this is principally because the temperature required to break down NH3 and N2H4 is at least 900xc2x0 C. However, the temperature can be reduced if irradiation with ultraviolet light is used conjointly. The wavelength of the ultraviolet light used at this time is 100-350 nm, and preferably 150-300 nm. It has been concluded that this is because such a high temperature as described above is not required since the NH3 and N2H4 are broken down by the ultraviolet light, and the same reactions as indicated above can proceed even on heat treatment at 300-700xc2x0 C., and preferably at 500-600xc2x0 C. Furthermore, the Sixe2x80x94OH bonds, Sixe2x80x94H bonds and the unpaired bonds in particular in a silicon oxide film which is being irradiated with ultraviolet light readily absorb the ultraviolet light and, as a result, a state of chemical excitation arises in these parts, and it is thought that this also promotes chemical reaction.
The effect is especially pronounced in those cases where the invention is applied to silicon oxide films which have been formed using the sputter method (and especially the silicon oxide films in which the oxygen concentration is less than the stoichiometric amount obtained with argon, for example, as the sputter atmosphere). That is to say, such a film has many unpaired bonds but, on heat treating in a hydrogen nitride atmosphere, such as an NH3 or N2H4 atmosphere at 300-700xc2x0 C., and preferably at 500-600xc2x0 C., while irradiating with ultraviolet light, the unpaired bonds are formed into nitrides and nitrogen is bonded instead of the oxygen which is deficient in terms of the stoichiometric ratio, and it is possible to form a silicon oxide film which has few unpaired bonds.
Such effects can also be obtained with silicon oxide films which have been formed with PVD methods other than the sputter method, or with various CVD methods. It is possible by using this invention in this way to reduce the number of unpaired bonds in a silicon oxide film which has been formed using a PVD method or a CVD method and to raise the nitrogen concentration while using a low temperature of 300-700xc2x0 C. Thus, TFT in which such a silicon oxide film is used as a gate insulating film exhibit excellent characteristics and high reliability.
If the treatment described in the third invention is carried out with a silicon oxide film which has been formed with a CVD method or PVD method then the Sixe2x80x94H bonds and Sixe2x80x94OH bonds in the silicon oxide film are converted to nitrides or oxides by the initial heat treatment in an N2O atmosphere, and they are converted to Sixe2x89xa1N or Si2xe2x95x90Nxe2x80x94O bonds, and the hydrogen content of the silicon oxide film is reduced.
Moreover, by means of the subsequent heat treatment in a hydrogen nitride atmosphere, the unpaired bonds (dangling bonds) which cannot take part in the reaction described above are converted to nitride or nitro-hydride and become stable.
The effect of the irradiation with ultraviolet light (wavelength 100-350 nm, and preferably 150-300 nm) in this present invention is very great. That is to say, the abovementioned reactions do not proceed at all in the absence of irradiation with ultraviolet light. A temperature of at least 900xc2x0 C. is required to realize such a reaction by means of pure heat treatment. That is to say, this is because the temperature required to break down N2O or hydrogen nitrides thermally is at least 900xc2x0 C.
However, the abovementioned reactions can be realized at lower temperatures by irradiating with ultraviolet light. It has been concluded that this is because in the first place such a high temperature as described above is not required since the N2O and hydrogen nitrides are broken down by the ultraviolet light, and the same reactions as indicated above can proceed even on heat treatment at 300-700xc2x0 C., and preferably at 500-600xc2x0 C.
Furthermore, the Sixe2x80x94OH bonds, Sixe2x80x94H bonds and the unpaired bonds in particular in a silicon oxide film which is being irradiated with ultraviolet light readily absorb ultraviolet light and, as a result, a state of chemical excitation arises in these parts, and it is thought that this also promotes chemical reaction. The reaction proceeds readily at the boundary between silicon oxide and silicon and, as a result, the nitrogen tends to be concentrated at the silicon oxidexe2x80x94silicon boundary.
The amount of nitrogen which is added and concentrated in the vicinity of the boundary with such a technique is at least ten times the average concentration for the silicon oxide film. The inclusion of 0.1-10 atom. %, and typically the inclusion of 1-5 atom. %, of nitrogen in the silicon oxide is desirable for a gate insulating film.
Consequently, the unpaired bonds and the Sixe2x80x94H bonds and Sixe2x80x94OH bonds which are weak bonds and easily broken by hot carriers at the boundary between the gate insulating film and the active layer are converted to Si N bonds and Si2xe2x95x90Nxe2x80x94O bonds, for example, which are strong bonds, and the change in the chemical state due to hot carriers is greatly reduced.
In this way, the durability with respect to hot carriers is improved by converting the unpaired bonds, Sixe2x80x94H bonds and Sixe2x80x94OH bonds in the silicon oxide film, and especially in the vicinity of the boundary with the silicon film, to nitride or oxide, and when used as a gate insulating film for a TFT, the effect is to improve the electric field mobility and the sub-threshold characteristic value (S value) and to prevent any falloff in the ON current (deterioration, change with the passage of time).
The effect is especially pronounced in those cases where the invention is applied to silicon oxide films which have been formed using the sputter method (and especially the silicon oxide films in which the oxygen concentration is less than the stoichiometric ratio obtained with argon, for example, as the sputter atmosphere). This is because the deficient oxygen can be supplemented by heat treating such a film in an N2O atmosphere and the composition of the silicon oxide film can be made to approach the stoichiometric ratio. The unpaired bonds which are not dealt with by the heat treatment in an N2O atmosphere are converted to nitride by means of a subsequent heat treatment in a hydrogen nitride atmosphere.
If a treatment as described in the fourth invention is carried out with a silicon oxide film which has been formed using a CVD method or a PVD method then the unpaired bonds, Sixe2x80x94H bonds and Sixe2x80x94OH bonds in the silicon oxide film are converted to nitride by the initial heat treatment in a hydrogen nitride atmosphere and converted to Sixe2x89xa1N or Sixe2x80x94Nxe2x95x90H2 bonds.
Moreover, the nitrogen hydride groups (NH2 groups for example), which are formed in the abovementioned reaction are converted to nitride or oxide by the succeeding heat treatment in an N2O atmosphere and form Sixe2x89xa1N bonds and Si2xe2x95x90Nxe2x80x94O bonds for example. The effect of the irradiation with ultraviolet light in the abovementioned reactions is very great, as in the case of the third invention.
The amount of nitrogen which is added to and concentrated in the vicinity of the boundary with such a technique is at least ten times the average concentration for the silicon oxide film. The inclusion of 0.1-10 atom. %, and typically the inclusion of 1-5 atom. %, of nitrogen in the silicon oxide is desirable for a gate insulating film.
In this way, the durability with respect to hot carriers is improved by converting the unpaired bonds, Sixe2x80x94H bonds and Sixe2x80x94OH bonds in the silicon oxide film, and especially in the vicinity of the boundary with the silicon film, to nitride or oxide, and when used as a gate insulating film for a TFT, the effect is to improve the electric field mobility and the sub-threshold characteristic value (S value) and to prevent any falloff in the ON current (deterioration, change with the passage of time).
The effect when the invention is applied to a silicon oxide film which has been formed with the sputter method (and especially a silicon oxide film in which the oxygen concentration is less than the stoichiometric ratio using argon, for example for the sputter gas), and the effect when it is applied to a silicon oxide film which has been formed with the plasma CVD method using a silicon source which contains carbon, such as TEOS for example, are the same as in the third invention.